The electronics industry has grown rapidly through technological advance and the current trend is toward miniaturization of circuits. For the emerging, high-speed, high-power devices such as microprocessors, ASIC's, and signal processors, packaging is a crucial issue. Such applications often require 2 to 10 watt power dissipation and speeds exceeding 100 MHz. Accordingly, many high-performance, low-cost package design options have been investigated. See, e.g., D. Mahulikar, A. Pasqualoni, J. Crane, and J. Braden, "Development of a Cost Effective High Performance Metal QFP Packaging System," in Proceedings of the IEEE International Symposium on Microelectronics, pp. 405-10 (1993).
Various design applications have required the use of gold in circuit fabrication. Gold resists corrosion, is chemically inert, is electrically and thermally conductive, and has a low ohmic contact resistance. This unique combination of properties allows gold to give circuits high efficiency by varying signals to and from various components and component arrays even when applied as a thin film (3-5 .mu.m thick). Consequently, gold is often deposited or coated on circuit lines and on different electronic components.
Gold can be deposited by various methods. To deposit gold from a solution containing metal salt, negative electrical charges are provided to convert the positively charged gold ion (by reduction) into the zero-valent state or the metallic form. In the usual case of electrolytic gold deposition, an external source of current provides the necessary charges for reduction at the cathode.
Alternative deposition methods do not depend on an external source of current. The charges required for deposition in these methods are supplied either by charge exchange reactions or are derived from chemical reducing agents. In charge exchange methods, a relatively less noble metal (usually the basis material) dissolves and the more noble gold ion in the solution is reduced and deposited on the substrate. Such methods are referred to as immersion or displacement deposition (or plating) processes.
In the case of chemical reduction methods, on the other hand, a suitable chemical compound (a reducing agent) supplies the necessary negative charges. The reducing agent is oxidized at the same time. Such methods are referred to as autocatalytic or electroless deposition methods.
Electroless gold deposition methods have become increasingly important in providing suitable metallurgy for electronic packaging applications. Such applications include contact areas, bonding surfaces on chip carriers (particularly ceramics), parts with glass-insulated bushings, transistor parts, cases, and many others. Electroless gold deposition methods play a critical role in simplifying the methodology of manufacturing ceramic-and polymer-based chip carriers, such as cavity pin grid arrays and surface mounted packages, and in enhancing design flexibility. See., e.g., M. Nakazawa and S. Wakabayashi, "Ceramic Packages and Substrates Prepared by Electroless Ni--Au Process," in Proceedings of the IEEE/CHMT Symposium, pp. 366-70 (1991).
The key challenge in cost and performance packaging technology is to provide high density multilayer interconnection capability with smaller wire bond pad spacing and conductor widths while retaining the design flexibility to achieve low impedance to output pins. Electroless gold plating technology offers unique advantages for the metallization of such structures.
Electrolytic plating requires extra circuit lines to connect pads together from layer to layer for connection to a tie (or bus or plating) bar. Often, after lamination, edge metallization is applied to the part so that, after firing, the part may be clipped onto the plating rack fixture for electrical contact. The plating rack is hung on a cathode bar for plating. The extra circuit lines and edge metallization can cause several problems. Extra circuit lines complicate circuit layout and cause cross-talk problems. Edge metallization must be removed by grinding or breaking. In addition, the different circuit line distances to the pad being plated cause plating thickness variations. Each pad will have a different electrical resistance from it to the tie bar and, because electrolytic plating thickness depends on current, the plating thickness will vary. Electrolytic barrel plating is used to avoid tie bars and shorting the circuit; parts are subject, however, to chipping and other damage in the barrel.
Electroless plating circumvents these problems with the electrolytic method. Because it does not require the ceramic circuitry to be shorted for electrical connection, unlike electrolytic plating, electroless plating does not require the entire metallized ceramic circuit to be shorted together and connected to a cathode with an electric current applied from an outside source to plate parts. Nor is it necessary to have extra conductor lines routed to the edge of the substrate. The electroless plating method is self-initiating upon placing the parts into a plating bath without having to apply an electric current.
Electroless plating eliminates plating bars, resulting in simplified circuit layout and reduced layout time; significantly reduces cross-talk due to extraneous plating conductors and circuitry; eliminates costly (and sometimes damaging) grinding and finishing operations to remove plating tie bars; provides improved gold plate thickness control on solder pads, wire bond fingers, and brazed components; and provides unique design opportunities for package configuration. B. Hassler, "Cofired Metallized Ceramic Technology and Fabrication Using Electroless Plating," in Proceedings of the International Symposium on Microelectronics pp. 741-48 (1986). Design flexibility and simplification of the circuit layout are critical factors in enhancing the performance of packaging modules.
Ceramic/polymer packaging modules with cavity die attach and gold wire bonding with pin grid arrays or surface mounted lead frames have become increasingly popular as single chip carriers for the I-486 and Power PC family of microprocessors. See, e.g., T. Goodman, H. Fujita, Y. Murakami, and A. Murphy, "High Speed Electrical Characterization and Simulation of Pin Grid Array Package," in Proceedings of the IEEE/CHMT Japan International Electronics Manufacturing Technology Symposium, pp. 303-07 (1993); D. Mahulikar, A. Pasqualoni, J. Crane, and J. Braden, supra. Molybdenum or tungsten is widely used within the alumina substrate as a conductor while copper is the metal of choice for polymer based chip carriers. The pad/pin assembly (Kovar/Cu--Ag or Ag) must be protected from corrosion and wet electro-migration by Ni/Au or Ni--Co/Au over-layers. (Kovar is an iron-nickel-cobalt alloy with a density of 8.3 g/cc, a thermal expansion coefficient (20.degree.-500.degree. C.) of 5.7 to 6.2.times.10.sup.-6, a thermal conductivity of 0.04 cal/cm-sec-.degree.C., and a specific electrical resistance of 50.times.10.sup.-6 ohm-cm.)
For pluggable pins, up to 10 .mu.m of heavy soft gold metallurgy is preferred. The wire bond pads and the cavity die attach areas are also plated with gold to provide suitable metallurgy for gold-silicon or JM 7000 epoxy die attach and gold or aluminum wire bonding. The gold should be 99.99% pure and conform to MIL SPEC 4520-C. Electroless gold plating processes using amineborane or borohydride as the reducing agent provide gold deposits of excellent quality able to satisfy these requirements.
In view of their advantages, a large number of electroless gold plating bath formulations are disclosed in the literature. See G. Ganu and S. Mahapatra, "Electroless Gold Deposition for Electronic Industry," in Journal of Sci. & Ind. Res., Vol. 46, pp. 154-61 (1987), and H. All and I. Christie, "A Review of Electroless Gold Deposition Processes," in Gold Bull., Vol. 17, pp. 118-27 (1984), for listings of various combinations of gold complexes and reducing agents which have been tested as potential electroless gold plating baths.
The electroless gold plating baths described in the literature, which use amineborane or borohydride as the reducing agent, contain gold in a cyanide complex with excess free cyanide as the stabilizer. The baths normally operate in the pH range of 12-14 and potassium hydroxide (KOH) is used to maintain the alkalinity. The typical deposition rate of these baths is about 0.5 .mu.m/hour. Lead or thallium is used to enhance the rate to about 2 .mu.m/hour. Both lead and thallium influence the quality of the gold metallurgy, however, and their concentrations must be kept very low (typically below 100 ppm) to avoid any adverse effect on bonadability. The concentrations of free cyanide and the lead or thallium are carefully optimized to provide adequate stability, good plating rate, and excellent metallurgy.
As plating progresses, the cyanide ion is continuously released and, with increasing free cyanide concentration in the bath, the plating rate drops considerably. Usually the plating solution is discarded (after about 4-5 hours) when the rate drops below 1 .mu.m/hour. Only about 25 to 35% of the gold content of the bath is used for plating. Thus, continuous bath operation for several hours is not possible. Furthermore, high volume production requires frequent new bath make-up and waste disposal--both of which increase the cost of processing.
The useful life of the electroless gold plating baths can be extended by replenishing the constituents of the bath. Replenishment procedures involving gold cyanide (AuCN) and potassium aurocyanide (KAu(CN).sub.2) have been attempted. See, e.g., Y. Okinaka and C. Wolowodiuk, "Electroless Gold Deposition: Replenishment of Bath Constituents," in Plating, Vol. 58, pp. 1080-84 (1971); F. Simon, "Deposition of Gold Without External Current Source," in Gold Bulletin, Vol. 26, pp. 14-26 (1993). Addition of gold cyanide resulted in excessive precipitation of gold particles, however, and bath decomposition after only a few hours of operation.
Moreover, to overcome the drop in the plating rate and to keep it around 2 .mu.m/hour, the concentration of rate enhancer must be steadily increased. Because rate enhancers affect the metallurgy of the deposition at higher concentrations, such replenishing solutions have very limited application in high volume manufacturing. The challenge remains, therefore, to develop a replenishing solution that will supply gold ions without increasing the free cyanide concentration in the bath. Such a procedure should not adversely affect the bath stability, plating rate, or the quality of the deposit metallurgy.
In summary, the literature discloses that no electroless gold plating bath has been established that is suitable for continuous production. See G. Ganu and S. Mahapatra, supra. Although some processes can be used on small scale applications with consistent success, the conventional electroless gold plating baths suffer from low deposition rates (about 1.5 .mu.m/hr), poor selectivity for conductor patterns and ceramics, short working lives, instability (mainly caused by nickel contamination), and poor adhesion to electroless nickel. See M. Nakazawa and S. Wakabayashi, supra. There remains a need, therefore, for a reliable electroless gold plating bath for wide-spread applications.
To overcome the shortcomings of such conventional baths, a new replenishing solution for a cyanide-based electroless gold deposition bath is provided. An object of the present invention is to provide an improved bath with increased stability. A related object is to assure relatively low self-decomposition of the reducing agent. Another object is to provide a bath able to deposit gold rapidly with a constantly maintainable deposition rate.
It is still another object of the present invention to reduce the tendency toward random deposition and increase the reproducibility of results (bath and deposit properties). An additional object is to provide a bath with long life (metal turnover) enabling repeated use of the bath chemistry while requiring simple maintenance. This will yield cost savings because the chemicals used to make up the bath can be conserved. In addition, chemical waste treatment and disposal of the cyanide solutions generated by the method of plating are reduced, thereby enhancing the price-performance factor for the electroless gold plating process. Yet another object of this invention is to provide a bath which has a low sensitivity to metallic contaminants (particularly nickel and tin ions).